Paris Gourtsoyannis

Police have appealed to the public for information to establish whether the terrorist behind Wednesday’s deadly Westminster attack had any accomplices.

On Friday, the death toll from the car and knife assault at parliament rose to four after a 75-year-old man, critically wounded when Khalid Masood drove at pedestrians on Westminster Bridge, died in hospital.

Investigators confirmed that Masood was born Adrian Russell Ajao and released the first official image of the attacker.

He is believed to have converted to Islam in the past 12 years following convictions for non-terrorist-related offences, and had been investigated by MI5 for extremism before falling off the intelligence radar.

Attention turned to the security of parliament, and whether the access point used by everyone from staff commuting by bicycle to the Prime Minister’s motorcade represents a weak point in the ring of steel around Westminster.

The imposing iron gates are left open for most of the day, and the three police officers that man the entrance and check the security passes of those entering are unarmed.

It has been claimed that Masood was shot dead by close protection officers from Defence Secretary Michael Fallon’s detail, rather than armed officers assigned to parliament.

Footage emerged late on Thursday showing the Prime Minister being ushered to a car in the moments after gunshots were heard in parliament. Her car was seen exiting the Westminster estate five minutes after Masood was shot dead.

Scotland Yard’s anti-terrorist chief Mark Rowley insisted that Parliament’s security arrangements were “proportionate” and had been developed over many years.

Mr Rowley said procedures for guarding Parliament had been designed so they were not “overly intrusive”.

“Our current arrangements have been developed with Parliament over many years and are designed to provide access to the seat of our government balanced with security that is proportionate but not overly intrusive,” he said.

“As would be expected, my team will work with Parliamentary authorities to assess whether a different tone is necessary.”

The streets around Westminster returned to normal yesterday with the last remaining restrictions on Parliament Square being lifted.

Westminster Tube station was also reopened. The return of traffic and crowds of tourists was accompanied by a growing number of floral tributes left beneath Big Ben, where Masood crashed his Hyundai 4x4, and at Carriage Gates, where he forced his way into parliament and launched a vicious knife attack on an unarmed police officer, PC Keith Palmer.

Amid the bouquets of flowers were pictures of Mr Palmer, messages from police forces across the UK, and Charlton Athletic football scarves – his favoured team.

Inside parliament, cordons sealing off New Palace Yard, where Mr Palmer was killed and Masood shot dead, were also lifted late on Thursday night following the end of forensic investigations.

Counter-terrorism detectives are attempting to trace any associates of Masood as they mount a huge investigation to establish what ­triggered his murderous rampage in Westminster.

It also emerged that three more people were arrested as part of the inquiry, taking the total up to 11. Six of those held were released without charge yesterday.

Asked about where Masood had been radicalised, Mr Rowley said: “Our investigation focuses on understanding his motivation, preparation and associates.

“Whilst there is no evidence of further threats, you will understand our determination to find out if he either acted totally alone, inspired by terrorist propaganda, or if others have encouraged, supported or directed him.”

Asked whether Masood had travelled overseas, the officer said: “We are looking at his history.”

Yesterday the Prime Minister issued a warning to internet giants that they “must do more” to stop extremist material being posted online.

It comes after the Islamic State terrorist group claimed to have inspired Masood’s attack, following a pattern seen in other so-called “lone wolf” terrorist atrocities across Europe.

Downing Street said the Prime Minister has been clear that companies like Google need to prevent terrorism and hate being spread in cyberspace.

Information on how to mount a terrorist attack was found to be easily accessible online in the wake of the Westminster atrocity.

Before the attack took place, Google had already been forced to promise it would take a “tougher stance” on hate content after an outcry and boycotts from advertisers over their content appearing alongside extreme material.

The Prime Minister’s official spokesman said social media companies and search engines must stop the material being posted and act quickly to take it down if it does appear online.

He said: “The fight against terrorism and hate speech has to be a joint one. The government and security services are doing everything they can.